Behind our anxiety is ignorance. Period. Ignorance is described as a veil that obscures the view of what is from so-called primordial awareness or Atman, if you wish - the aspect of the whole (Brahman) in us. (The Buddha explicitly rejected the notion of Atman, but numerous later Indian (tantric) writers messed everything up). It is like continuous day-dreaming. The meaning of Awakening is literal awakening from this habitual delusional day-dreaming to what is.
Most of our fears are due to misapprehension of reality, like classic mistaking of a rope for a snake, and our attaching to (and hence the fear of losing) that illusory my self, which is nothing, but an appearance to ignorant and confused introspection.
This crude formulation is as old as Upanishads.